I don't think that's quite right. As I understand the law (and IANAL), you could still get their patent invalidated for lack of novelty — you just couldn't claim the patent yourself in that case.
Prior art still invalidates patents in first-to-file systems.
First to file only means that, if you filed for a patent on some thing after someone else who also filed to patent it, you definitely don't get the patent (in a first-to-invent system, whichever of you can show the oldest documentation of invention gets the patent).
Under a first to file system, you still have to be an inventor. If you're trying to patent something that you copied from someone else, you're not an inventor.
And even if you didn't derive your work from theirs, you still have a problem with prior art.
You run that risk with the previous system. That has not changed. The first-to-file rule change only applies when the USPTO needs to resolve a conflict between two similar patents that are applied for around the same time.
The patent system has never protected inventors that do not attempt to patent their invention.